The word was originally for large, badass guys who travel with you in the Australian bush and kick ass for you if anything, man or beast, gives you any problem.
Outback guy sitting on log drinking beer: Cheers Mate!
Badbuy: Shouldn't you have bought a gun instead of beer?
Outback guy: No, I don't need a gun I got a Donk.
Badguy seems perplexed: Whats a Donk?
Donk right before he knocks him out with one punch: